Compound DC electric motors have at least two field windings with one being in series with the armature winding and the other being in parallel therewith. In many of such compound motors the series field winding has a relatively small number of low resistance turns in comparison with the shunt field winding. Thus, the shunt field winding has a comparatively long time-constant. Consequently, when electrical power is first applied to such motor, a heavy current flows through the armature and series field winding circuit prior to the time that normal current flow can be established in the shunt field winding. As the two field windings are closely coupled inductively, the relatively high current in the series field winding circuit tends to momentarily induce a reversed current flow in the shunt field winding. This contributes to extremely large current surges flowing through the windings and other conductors including the brushes. These current surges during start-up cause severe heating, brush deterioration, power wastage, may interfere with proper commutation, and in general tend to reduce the operational life of the motor. If the motor is being operated from a battery, the abnormal current may also cause premature battery deterioration.
The present invention is directed to overcoming one or more of the problems as set forth above.
According to the present invention, a control system for a compound DC electric motor having a power source comprises a series field winding, a shunt field winding connected in parallel with the series field winding, and means for connecting the shunt field winding and series field winding with the power source. Time delay means is positioned between the series field winding and the power source for energizing the series field winding a predetermined time period after the energizing of said shunt field winding.